During a metal cutting operation, any vibratory motion between a cutting tool and workpiece may lead to non-beneficial cutting performances. Furthermore, such vibration may cause the cutting tool or the machine tool to become damaged. Excessive self-excited vibrations, frequently called “chatter,” between the cutting element of a machine tool and the surface of the workpiece cause poor surface finish, tool breakage and other unwanted effects that plague machining operations. Such vibrations arise especially when the tool includes a long unsupported length that will permit deflection of the tool. When chatter does occur the machining parameters must be changed and, as a result, productivity may be adversely affected.
When using the stiffest and most advanced machine tool design, there still remains vibration in the machine tool. For example, a long cantilevered boring bar with a single cutting element at its free end will, by its interaction with the workpiece, tend to vibrate. This vibration may be considered to be self-excited because it is generated by the interaction of the cutting element and the workpiece. Such self-excited vibration occurs at frequencies near the natural frequency of the cantilevered tool and tends to increase in amplitude under small disturbing forces.
One such example of a tool that may encounter issues due to excessive vibration is boring bars. Boring bars are used to fabricate deep holes. A primary difficulty in their use is that, because the holes tend to be deep and narrow, the boring bars must be long and have small diameters. Therefore, during machining, the variable cutting force causes the tool to deflect and leave a wavy surface behind. When the cutting edge encounters this wavy surface in the next revolution, additional forces and deflections may be caused which may lead to chatter or unstable machining. The results of chatter are poor surface finish and hole accuracy, large forces and deflections, and potential damage to the tool, workpiece, and/or machining center.
To reduce these vibrations, various methods have been employed. In one example, the metal removal rate may be decreased. However, this approach interferes with production and only minimally reduces the amount of vibration.
Attempts to eliminate the vibration in the boring bar may also include using a boring bar fabricated from solid carbide. However, solid carbide is extremely expensive. Furthermore, solid carbide is fairly brittle and a minor impact upon the boring bar during use or setup may inadvertently damage the bar.
A further attempt to reduce vibration in boring bars includes mounting upon or within the bar a dynamic vibration absorber, such as that absorber disclosed in U.S. Pat. No. 3,774,730, which is comprised of a cylindrical mass of a high density material supported on rubber bushings. When optimally tuned, the mass oscillates in response to vibration produced in the boring bar to cancel out vibration. The absorber may be tuned to accommodate the boring bar for the speed at which the workpiece or boring bar is rotating, the length of the boring bar and the type of machine tool connected at the end of the bar. Such an adjustment is made by longitudinally urging pressure plates at opposing ends of the cylindrical mass thereby compressing the rubber bushings against the mass which simultaneously shifts the position of the mass and alters the stiffness of the rubber bushings to change the dynamics of the cylindrical mass.
However, even with such a design available, each time the boring bar is to be used under different conditions, it must be tuned using sophisticated equipment that may or may not be available on the shop floor. Additionally, in many instances of using dynamic absorbers with boring bars, the boring bars are hollowed and a spring-mass-damper system is added inside the boring bar structure. This places limits on the boring bar diameter, which must be large enough to contain the dynamic absorber, and increases the boring bar cost.
Accordingly, it would be beneficial to provide a holder system for tools that is capable of reducing chatter and/or unstable machining without the need for complex instrumentation. It would also be beneficial to provide a holder system for tools that is capable of reducing chatter and/or unstable machining that may be used without modifying the structure of the tool. It would also be beneficial to provide a holder system for tools that is capable of reducing chatter and/or unstable machining that may be used with existing tools.